docs: def _save_notebook(self, os_path, nb):
          """Save a notebook to an os_path."""
          with self.atomic_writing(os_path, encoding='utf-8') as f:
              if ftdetect(os_path) == 'notebook':
                  nbformat.write(nb, f, version=nbformat.NO_CONVERT)
              elif ftdetect(os_path) == 'markdown':
                  nbjson = nbformat.writes(nb, version=nbformat.NO_CONVERT)
                  markdown = convert(nbjson,
                                     informat='notebook',
                                     outformat='markdown',
                                     strip_outputs=self.strip_outputs)
                  f.write(markdown)